Servo system for disk-flutter in rotating storage systems

ABSTRACT

In order to handle platter flutter in a hard disk operated at high speed (e.g. 10,000 RPM) and maintain tracking of read/write head, peak filters are added to a head tracking servo feedback loop. In some embodiments, several lag-lead type peak filters centered at a plurality of flutter mode frequencies are provided to process a Position Error Signal (PES) read from the read/write head. The several filter are preferably arranged in series (i.e. to operate on the PES sequentially), alternatively the filters can be arranged in parallel. Phase lags and leads caused by the several filters partially cancel at frequencies intermediate center frequencies of the several filters. According to another embodiment two or more, preferably three narrow band filters are used to cover the spectrum of each flutter mode. In a configuration mode, a spectrum of the flutter modes can be obtained and used to set the center frequencies, and bandwidths of filters used to filter out the flutter modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed broadly relates to data storage devices. Moreparticularly, the present invention relates to techniques for correctingthe adverse effects of vibrations on servo systems in data storagedevices.

2. Description of the Related Art

Hard disk drives are commonly used for mass storage purposes incomputers. In FIG. 1 there is shown a block diagram of the majorelectro-mechanical components of a disk drive 102. The disk drive 102has a read/write transducer 104, voice coil actuator 106, recordingmedium or disk 108, and read/write control electronics 110. There is aconstant demand for increase storage capacity and reduced disk accesstimes as new software and new applications become available. The presentgeneration of hard disk drives 102 are designed to operate in aportable, a desk-top and a server environment. In order to meet thedemand for lower disk access times, the spindle rotation speed for disk108 has been on the increase. However this increase in spindle rotationspeed has present substantial design challenges to overcome.

These design challenges can be categorized based on their source and thedirectional effects on the read/write transducer 104. The first problemis vibration which is caused from mass imbalances, physical tolerancelimitations and electro-mechanical sources. The vibration manifestsitself by moving the read/write transducer 104 back and forth in adirection parallel to the surface of the disk 108. The typical sourcesof vibration include the disk drive itself, other disk drives and otherelectro-mechanical drives sharing a chassis such as CD ROM drives,diskette drives, and tape drive devices. Vibration produces poor settleout and/or reduces track following characteristics, especially for trackdensities of 10,000 tracks per inch (TPI) and beyond. Servo systemsreduce the track-follow error by about 20 to 30 dB using basic servoloop error rejection properties. Another technique for reducing theeffects of vibration is disclosed in application Ser. No. 08/119,181 byS. M. Sri-Jayantha et al. entitled “Adaptive Vibration Control For ServoSystems In Data Storage Devices” filed on Jul., 20, 1998 and is commonlyassigned herewith to IBM.

The second problem is flutter of the disk platter 108. The flutter isthe movement of the disk 108 that causes track mis-registration error(TMR). Disk-flutter, is primarily caused by pressure fluctuationsassociated with internal turbulent airflow. The airflow is the result ofaerodynamic effects between the fast rotating surface of the disk 108causing air to disturb the close flying read/write transducer 104. Asthe rotating speed of a disk is increased from 5,400 rpm to 10,000 rpm,the aerodynamically induced disk-flutter becomes a major contributor totrack mis-registration error. The track density of present generationdrive is about 15,000 TPI. About 30% of the TMR budget is consumed bydisk-flutter effects at disk speeds 7,200 rpm in a disk drive withconventional servo-mechanics configuration. As track density isincreased, the disk-flutter-based TMR is expected to contribute wellabove 30% of the allowed TMR budget unless a cost effective solution isfound.

The impact of disk-flutter on TMR can be minimized by aerodynamicredesign of the base-plate, improved stiffness and damping of diskplatter substrate, or by novel servo method. Present 3.5″ disk driveshave reached the TMR limit posed by the disk-flutter mechanics.Disk-flutter has been observed in the high track density 3.5″ products.The disk-flutter problem can be tackled from three technical viewpoints:mechanical, aerodynamic, and servo.

A higher bandwidth servo system can effectively compensate for thedisk-flutter, but no cost effective methodology has been proposed by thestorage industry to increase the servo bandwidth without increasing thecomponent count. The strength of the airflow disturbance can be reducedby means of shrouding. However, complex shrouding or machiningoperations makes the mechanical approach not economical. Moreover, theshrouding must be custom designed for each type and model of hard diskdrive 102. One approach can be found in the prior art has been to reduceairflow disturbance in the disk enclosure is in U.S. Pat. No. 4,583,213by Allen T. Bracken et al. for “Air shroud for data storage disks”,issued Apr. 15, 1986. Accordingly, a need exists to reduce fluttering indisk drives without the need of redesigning custom shrouding or custombase plates.

Another approach to reduce disk-flutter is by use of an alternate diskplatter substrate with increased stiffness and damping properties.However, new substrates call for investments in research anddevelopment. Therefore, a need exists for a method and apparatus toreduce the effects of disk-fluttering TMR without the need of new disksubstrates.

Still another approach to reduce disk-flutter problems is to use smallerdiameter disk drives. Due to the lack of cost effective solutions forlarger diameter hard disks, the storage industry has moved towards 3.0″and 2.5″ diameter disks to minimize the severity of disk-flutterproblems. The effect of mechanical movement of the data tracks due todisk-flutter can be effectively track-followed by increasing the servobandwidth of a head positioning system. Using micro actuators thebandwidth of a conventional head positioning system can be increased.Examples of micro actuators are found in U.S. Pat. No. 5,657,188 by RyanJurgenson et al. for “Head suspension with tracking microactuator”issued Aug. 12, 1997 and U.S. Pat. No. 5,189,578 by Kenji Mori for “Disksystem with sub-actuators for fine head displacement” issued Feb. 23,1993. But using micro actuators can add cost to the servo assembly.Accordingly, a need exists to reduce the effect of disk-fluttering TMRwithout the need to reduce the diameter of the disk and the need to usenew forms of disk actuators.

SUMMARY OF THE INVENTION

Briefly, in accordance with the present invention, a rotating mediastorage system comprising: a servo feedback loop for providing aposition error signal to position read/write transducer over a rotatingmedia storage; a detector for detecting at least one of the disk-fluttermodes induced on the transducer and a narrow band filter is set to servocompensate the fundamental mode of at least one flutter mode.

In another embodiment, the disk-flutter mode is minimized using alead-lag filter which reduces the amplification of the disk-flutter inthe flutter enhancement zone.

In accordance with another embodiment of the present invention, a methodthat corresponds to the above rotating media storage system isdisclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the major electro-mechanical components ofa disk drive.

FIG. 2 is a block diagram of the disk drive in FIG. 1 with the servosystem tools used to evaluate and implement the disk-flutter servoaccording to the present invention.

FIG. 3a is a frequency domain plot of the PES power spectrum includingpeaks of five disk-flutter modes peaks using the servo tools of FIG. 2with the product servo on according to the present invention.

FIGS. 3b-f are time domain plots of the five different peaks for eachflutter mode of the PES signal in FIG. 3 as filtered through a 6 dBsecond order digital peak filter tuned for each frequency.

FIG. 4a is a frequency domain plot of the frequency stability of aflutter mode using overlapping time traces of the PES for the firstflutter mode of FIG. 3a as filtered by a 6 dB peak filter for 10revolutions of the disk drive, according to the present invention.

FIG. 4b is a frequency domain plot of the frequency stability of aflutter mode using overlapping time traces of the PES for the firstflutter mode of FIG. 3a as filtered by a 6 dB peak filter for 30revolutions of the disk drive, according to the present invention.

FIG. 5a is a frequency domain plot of the PES power spectrum of the diskdrive of FIG. 2 without any filtering using the disk-flutter servo,according to the present invention.

FIG. 5b is a frequency domain plot of the PES power spectrum of the diskdrive of FIG. 2 with a single 6 dB peak filter set at 501 Hz in serieswith the disk-flutter servo, according to the present invention.

FIG. 6a is a time domain plot of the PES corresponding to the PESfrequency trace in FIG. 5a without any filtering, according to thepresent invention.

FIG. 6b is a time domain plot of the PES corresponding to the PESfrequency trace in FIG. 5b with a single 6 dB peak filter set at 501 Hzfilter, according to the present invention.

FIGS. 7a-b is a set of frequency domain plots for an open-loop transferfunction of the magnitude and phase plots for a disk-flutter servohaving two 6 dB peak filters to compensate for a simulated 501 Hz and617 Hz disk-flutter components, according to the present invention.

FIG. 8 is a frequency plot detailing the transfer function of FIG. 7,according to the present invention.

FIG. 9a is a plot of the estimated change in TMR due to drift in nominalgain of the disk-flutter servo system in FIG. 7, according to thepresent invention.

FIG. 9b is a plot of the sensitivity of the disk-flutter servo in FIG. 7due to system gain uncertainty, according to the present invention.

FIG. 10 is a plot of the effect of flutter mode frequency shift on TMRfor a fixed flutter-servo system in FIG. 7 according to the presentinvention.

FIG. 11 is a block diagram showing one procedure for determining thecenter frequency of a disk-flutter mode using a digital sweep method,according to the present invention.

FIG. 12a is a plot of the results using the digital sweep method of FIG.11 with a peak filter of 20 dB, according to the present invention.

FIG. 12b is a plot of the results using the digital sweep method of FIG.11 with a peak filter of 6 dB, according to the present invention.

FIG. 13 is a block diagram of one embodiment of the disk-flutter filterfor 502 Hz and 617 Hz with a standard product servo in parallel form,according to the present invention.

FIG. 14 is a block diagram of another embodiment of the disk-flutterfilter for 502 Hz and 617 Hz with a disk-flutter servo in series filterform, according to the present invention.

FIG. 15 is a frequency domain plot of the conventional product servo ofFIG. 2 as compared with the computed error rejection transfer functionof a di-flutter servo having four filters for disk-flutter TMR,according to the present invention.

FIG. 16a is a set of time domain and frequency domain plots for theflutter spectrum at 512 Hz showing the effect of fixed frequencyamplitude modulation with the fundamental harmonic, left side band andright side band preserved, according to the present invention.

FIG. 16b is a set of time domain and frequency domain plots showing theeffect of fixed frequency amplitude modulation with no fundamentalharmonic and the left side and right side band preserved, according tothe present invention.

FIG. 16c is a set of time domain and frequency domain plots showing theeffect of fixed frequency amplitude modulation with only the left sideband preserved, according to the present invention.

FIGS. 17a-c are sets of time domain and frequency domain plotscorresponding to FIG. 16a-c respectively, illustrating the effect ofrandomized frequency modulation, according to the present invention.

FIG. 18 is a set of frequency plots for the amplitude and the phase ofthe disk-flutter servo comparing the use of three narrow 10 dB filtersversus the use of a single 6 dB filter according to the presentinvention.

FIG. 19a is a frequency domain plot of the power spectrum correspondingwith FIG. 18 without any filtering using the product server, accordingto the present invention.

FIG. 19b is a frequency domain plot of the power spectrum of the PES ofthe disk drive of FIG. 2 with a three 10 dB peak filter set at 501 Hz inseries with the product servo, according to the present invention.

FIG. 20 is a block diagram of an embodiment for determining the centerfrequency of a disk-flutter mode as in FIG. 11 but with the ability todetermine bandwidth of the fundamental mode and each of thecorresponding side bands, according to the present invention.

FIG. 21a is a block diagram of a lag-lead filter of an embodiment of thedisk-flutter servo system in FIG. 20 with a lag-lead compensator.

FIG. 21b is a frequency domain plot of the transfer function of a servosystem with flutter mode components in the range of 500-1000 Hz withouta lag-lead filter as shown in FIG. 21a, according to the presentinvention.

FIG. 21c is a frequency domain plot of the transfer function of a servosystem of FIG. 22b with flutter mode components in the range of 500-1000Hz with a lag-lead filter in position as shown in FIG. 21a, according tothe present invention.

FIG. 22 is a block diagram of the major electrical components of aninformation processing system in which the flutter-servo system maybeimplement, in accordance with the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

This description is divided into two section. The first sectiondescribes how the disk-flutter modes can be captured and analyzed for aspecific hard disk 2. The second section describes how the disk-flutterfiltering is constructed to minimize the effects of the disk-fluttermodes in the first section. The drawings referred to throughout thisspecification, use like numeral to refer to like parts throughoutseveral views.

A. Disk-Flutter Mode Measurement and Analysis

FIG. 2 is a block diagram of the disk drive in FIG. 1 with a servosystem tools used to evaluate and implement the disk-flutter servoaccording to the present invention. The disk-flutter tool consists oftwo parallel independent servo loops. The first servo loop is a productservo feedback loop 204 which is typically part of the controlelectronics 110 for the servo actuator 202. A position error signal(PES) 206 is read from the servo actuator 202. Following servoprinciples known in the art, the PES 206 is used to correct andcompensate for the position of the actuator 202. The first servo loopsallows the control electronics 110 to operate without any externalsupport, but the PES 206 can be read into a separate digital signalprocessor in real time for data analysis purposes. The second servo loopalso is feed by the same PES 206. The disk-flutter servo 208 is shown intwo sections, a DSP (digital signal processor) section 212 and adisk-flutter servo section 210. The disk-flutter servo 208 canaccurately replicate the transform and filter characteristics of theproduct servo 204 so as to take the product servo 204 out of the loopduring analysis if desired. The DSP section 212 can be used for analysisonly or in conjunction with the disk flutter servo section 210 to form acomplete disk flutter servo 210 to replace the product servo 204. Thesecond servo loop allows custom implementation of a novel servo usingthe same identical PES stream while the product servo loop 204 isdisabled. By means of this setup, a strict comparison betweenconventional servo performance and novel servo performance can be madeon the identical hardware for disk drive 102. A DSP algorithm residingin DSP 210 provides the ability to implement a variety of narrow-banddigital filters tuned at a given frequency so that the time domaincharacteristics of the individual disk-flutter induced PES component canbe analyzed. The following plots are obtained using the PES signal 206displayed on spectral equalizer 214.

FIG. 3a. is a frequency domain plot of the PES power spectrum of peaksfor five disk-flutter modes using the servo tools of FIG. 2 with thedisk flutter servo on, according to the present invention. The Y-axisrepresents the PES spectrum in bits squared. One track width of harddisk platter 108 is equal to an 8 bit word. Therefore, there are 2⁸ or256 bits equal to one track width. The graphs are normalized so thatthere are +/1 127 bits centered around a center line of Zero. The harddisk has a several significant flutter modes. Five significant fluttermodes at 501 Hz, 617 Hz, 737 Hz, 829 Hz and 980 Hz are shown in FIG. 3a.These flutter modes are labeled 1 through 5 with the flutter modes 1 and2 highlighted for emphasis by a box 302. FIGS. 3b-f depict time domainplots of the five different peaks for each disk-flutter mode of the PESsignal in FIG. 3 as filtered through a 6 dB second order digital peakfilter tuned for each frequency. Filters with higher order filters maybe used for this disk-flutter analysis that would more clearly demarcatedisk-flutter peaks, but the second order filters are throughout thisdescription for simplicity. Conventionally, the flutter data is viewedin a frequency domain as shown in FIG. 3a, where the five spectral peaksat 501 Hz, 517 Hz, 737 Hz, 820 Hz and 980 Hz have about 50 Hz spectrumspread. Because of this broad spectral characteristics as shown in thefrequency domain, a broad filter would produce excessive phase losses inthe product servo-loop 204 thus causing instability or degradation inTMR. The significance of the time domain data in FIGS. 3b-3 f is that itis amplitude modulated. Even though the disk-flutter components do nothave a steady amplitude a detail analysis shows that the frequency ofeach flutter mode is stable and does not shift substantially underconstant operating temperature, this is detailed further in FIGS. 4a-bbelow. Therefore the fundamental frequency component of flutter TMR canbe servo compensated, regardless of the amplitude modulation that causesthe broadening of the frequency spectrum, by a relatively narrow bandfilter with its peak frequency selected to match that of eachdisk-flutter component. A more detail discussion, below in Section B“Disk-Flutter Servo Construction” is provided to distinguish theamplitude modulation behavior of each flutter-component and additionalopportunities available to enhance the disk-flutter servo 208.

FIG. 4a is a frequency domain plot of the frequency stability of aflutter mode using overlapping time traces of the PES for the firstflutter mode of FIG. 3a as filtered by a 6 dB peak filter for 10revolutions of the disk drive, according to the present invention. FIG.4 is a frequency domain plot of the frequency stability of a fluttermode using overlapping time traces of the PES for the first flutter modeof FIG. 3a as filtered by a 6 dB peak filter for 10 revolutions of thedisk drive, according to the present invention. For the X-axes, onerevolution of the disk platter 108 is equal to about 66 tracks whichtakes about 8.5 milliseconds. So time or rotations can be usedinterchangeably in these plots. It can be seen that for 10 and 30revolutions of the repeated traces the frequency feature is ratherconsistent even though not fixed. Hence, the use of a narrow band peakfilter running in cascade or parallel with the conventional trackfollowing controller provides an opportunity that was not obvious beforeto servo compensate against the disk-flutter mode. It will also beapparent to those skilled in the art that a broad band peak filter willproduce excessive phase lag in the open-loop crossover region, and woulddestabilize the conventional track follow controller.

B. Disk-Flutter Mode Filter Construction and Analysis

Having identified the that the disk-flutter mode has a plurality ofpeaks within the frequency domain, but that each frequency component israther consistent over the time domain, the filter construction begins.FIGS. 5a and 5 b shows the power spectrum of PES with and without asingle 6 dB peak filter configured in parallel to the track-followcontroller. From the spectral plot it can be seen that the peak filterat 501 Hz does indeed removes the flutter component corresponding tothis frequency.

FIG. 5a is a frequency domain plot of the PES power spectrum of the diskdrive of FIG. 2 without any filtering using the disk-flutter servo 208.FIG. 5b is a frequency domain plot of the power spectrum of the PES ofthe disk drive of FIG. 2 with a single 6 dB peak filter set at 501 Hz inseries with the disk-flutter servo 208. The effectiveness of a servosolution to disk-flutter is see by this 6 dB filter. This effectivenessis further shown in the corresponding amplitude time domain plots. FIG.6a is a time domain plot of the PES corresponding to the PES frequencytrace in FIG. 5a without any filtering. FIG. 6b is a time domain plot ofthe PES corresponding to the PES frequency trace in FIG. 5b with asingle 6 dB peak filter set at 501 Hz filter. When the filtering isrepeated using two peak filters configured centered at 501 Hz and 617Hz, and corresponding enhancements in the frequency domain are seen (notplotted). All peak filters are know to effect the phase characteristicby providing a lead and then a lag component around the flutterfrequency. A conventional product servo 204 provides about 30 degreesphase lead. It becomes apparent, that a 20 dB or 30 dB broad band filterwill not work because of the excessive lag that causes instability ofthe product servo 204. If a filter designer over compensates, there canbe instability to the phase and if the filter under compensates, therewill be no effect on the disk flutter modes. These filters in FIG. 6 areoptimally tuned. Recall as determined above in section A “Disk-FlutterMode Measurement and Analysis” above, in the hard disk system 102, theflutter dynamics consists of a multitude of frequency components, anddoes not occur at a single frequency. The disk drive 200 analyzed hadmarginal servo transfer function phase properties due to other systemconstraints, and the use of several disk-flutter mode peak filtersproduced excessive transfer function distortion in the neighborhood ofthe flutter frequency region. Accordingly this distortion limits the TMRmeasurement in terms of 1-sigma PES. Therefore, in order to determinethe viability of servo compensating more than one flutter frequencycomponent, a simulation method in which a conventional product servo 204having 40 degree phase margin is used in the subsequent analysis todemonstrate the principle of control disk-flutter modes with narrow bandfilters.

FIGS. 7a-b is a set of frequency domain plots of an open-loop transferfunction of magnitude and disk phase portions for a disk-flutter servohaving two 6 dB peak filters to compensate for a simulated 501 Hz and617 Hz disk-flutter components. Note that the narrow peakcharacteristics illustrated at 180 Hz is irrelevant for thecharacterization of the flutter servo modes. The narrow peakcharacteristics at 180 Hz is used to compensate for spindle harmoniceffect which is not associated with disk-flutter according to thepresent invention.

Turning to FIG. 8 is a frequency plot detailing the transfer function ofFIG. 7. Observe that in order for the filters to produce minimum sideeffect, the phase lead of the conventional controller should be above 40degrees nominally. The side effect refers to undesirable distortion ofthe nominal rejection transfer function that could amplify other PEScomponents inadvertently in other regions in the frequency domain. Forexample, notice that the disk flutter modes occurs at about 600-700 Hz.This is precisely the area of cross-over of the zero axis for themagnitude. In general, with any filter, the larger the gain, the largerthe growth in phase margin. To remain stable, the zero magnitudecrossing of a filter design must have a phase above −180 degrees. Forhigher flutter frequencies in the range of 800-900 Hz, the phase marginbegins to droop below the −180 degree line and the disk drive system 102become unstable. Accordingly, the design is to tackle only the first twoflutter modes to reduce the chances of injecting instabilities throughreduction in phase margin for high disk-flutter frequencies.

FIG. 9a is a plot of the estimated change in TMR due to drift in nominalgain of the disk-flutter servo system in FIG. 7, according to thepresent invention. Notice the improvement in the 1-sigma TMR of 8.5 PESbits as compared to 4.5 bits when the disk-flutter servo for 501 Hz and617 Hz are added. FIG. 9b is a sensitivity plot of the disk-flutterservo of FIG. 7 due to system gain uncertainty. About 10% degradation intotal 1-sigma TMR is observed for a 20% drop in gain, and only a 2%degradation is seen for a 20% increase in gain. In other words, theopen-loop gain changes on a product to product basis. As gain goes up to+/−5 to 10% in variation, which is typical in a disk servo producttolerances, the sigma levels of the errors does not get excessivelyaggravated and in fact improves if the gain is increased. It should benoted that the sensitivity estimates are highly dependent on theproperties of the disturbance spectrum.

FIG. 10 is a plot of the effect of flutter mode frequency shift on TMRfor a fixed flutter-servo system in FIG. 7. For a shift of +/−50 Hz inthe both 501 Hz and the 617 Hz, the TMR degradation of 13% to 45% ismeasured. Therefore it is important to have an accurate estimate of theflutter frequency for each hard disk 102 product type and for each disk108 or platter so that the peak frequency can be set accordingly. Thedisk-flutter system servo designer must use well timed calibration runsto keep track of each flutter mode frequency if optimum TMR values areto be secured.

FIG. 11 is a block diagram showing one procedure for determining thecenter frequency of a disk-flutter mode using a digital sweep method,according to the present invention. When the disk drive 102 is put intrack following mode, the PES 206 stream is passed through a digitalfilter 1102 whose peak frequency is variable. The peak frequency of thedigital filter 1102 is gradually increased from a minimum frequencyvalue up through a maximum. The filter peak output amplitude ismemorized in a table as part of the spectrum analyzer (not shown) whichis depicted by the graph 1104. In the example used, the peak amplitudeoccur around 502 Hz and 617 Hz. These peak amplitudes can be determinequickly be knowing the measured open-loop nominal values of the filterpeaks for the servo and the nominal servo drift values. Typical driftvalue are +/1 50 to 100 Hz and other values are contemplated. Theresolution of the frequency measurement in the spectrum analyzer can beadjusted by means of step size and time record length used to generatethe spectral table. Typical step sizes are 10 Hz but other sizes arepossible. FIG. 12a is a plot of the results using the digital sweepmethod of FIG. 11 with a peak filter of 20 dB and FIG. 12b iscorresponding plot of the results with a peak filter of 6 dB. Each tracecorresponds to different time length as denoted by number of revolutionsused to compute each data point. It can be seen that a 20 dB peak filterwith 300 revolution long data record converges to 500 Hz and 620 Hz,very close to where the flutter components are located. Improper choiceof the detection filter peak gain can produce poor results as shown inFIG. 12b in which the spectral peaks are averaged out along with othernoise components in the PES 206 stream.

Two disk-flutter servo architectures are available based on theplacement of disk-flutter filters relative to the product servo 210. Oneimplementation is a parallel implementation and the other implementationis a series implementation. FIG. 13 is a block diagram of one embodimentof the disk-flutter filter for 502 Hz and 617 Hz with a standard productservo according to the present invention in parallel form. FIG. 14 is ablock diagram of another embodiment of the disk flutter filter for 502Hz and 617 Hz with a disk-flutter servo 208 in series filter form. Theparallel filter 1302 may be used for other filters such as the describedin Ser. No. 08/119,181 by S. M. Sri-Jayantha et al. entitled “AdaptiveVibration Control For Servo Systems In Data Storage Devices” filed onJul., 20, 1998 and is commonly assigned herewith to IBM. It was foundthat when the peak gain is about 6 dB a series realization alwaysproduces 6 dB peak gain enhancement whereas a parallel implementationmay require prior knowledge of the conventional track follow productservo 204 properties. Otherwise the coupling effect of two parallelsystem (i.e., conventional product 204 servo and a peak filter) couldproduce undesirable open-loop transfer function characteristics.Therefore a modular design of conventional track following controllerand peak filters for a disk-flutter servo 208 may not be not beeffective in parallel realization and appropriate caution must beobserved. However in cases where the peak gain is substantially higherthan the corresponding conventional servo gain at the frequency, theneither the series or parallel implementation can be implemented.

FIG. 15 is a frequency domain plot of the conventional product servo ofFIG. 2 as compared with the computed error rejection transfer functionof a flutter servo having four peak filters for disk-flutter TMRincluding a peak filter around 240 Hz. As illustrated, the use of peakfilters does indeed improve the rejection levels at and around theflutter frequencies. This point is important, the improvement inrejection is at the flutter-mode frequency. The phase loss due to onepeak filter is balanced by the phase lead provided by the adjacentfilter. Hence only the last, that is, the highest frequency filter isthe one that produces an undesirable phase loss resulting in the loss ofrejection gain. This effect allows the rejection transfer function torise above the conventional levels at higher frequencies in the range of1.2 to 3 KHz. Thus, if that hard disk drive 102 does not have anysignificant disturbance components at this higher frequency range then aclear advantage is achieved by the disk-flutter servo as demonstrated byFIG. 15. On a selective frequency basis, it may be better to employ thisembodiment.

In another embodiment, a different filter design is used when the phaseloss of the disk-servo system 208 is required to be minimizedconsideration is shown. This result is shown in FIG. 16, which depicts aset of time domain and frequency domain plots for the flutter spectrumat 512 Hz showing effect of fixed frequency amplitude modulation. Hereis a demonstration of the time domain and we are determining what theamplitude modulation means in light of the known literature on servosystems. In FIG. 16a notice the flutter spectrum in the frequency domainhas three components: (i) a fundamental harmonic; (ii) a left side band;and (iii) a right side band. FIG. 16b is a set of time domain andfrequency domain plots showing effect of fixed frequency amplitudemodulation with no fundamental harmonic and the left side and right sideband preserved. FIG. 16c is a set of time domain and frequency domainplots showing effect of fixed frequency amplitude modulation with onlythe left side band preserved. The actual flutter mode component, asshown in FIGS. 3b-f, has a mixture of modulation frequency and thestrength of each modulation frequency appears to be time varying. Thisobservation is simulated by generating a randomly changing modulationfrequency pattern. FIGS. 17a-c are a set of time domain and frequencydomain plots corresponding to FIGS. 16a-c respectively, illustrating theeffect of randomized frequency modulation. Observe that the measuredflutter mode spectrum of FIG. 5a could fit into any one of these plotsillustrated in FIG. 17. The point is simple. Instead of using a singlerelatively broad band peak filter to cover the full bandwidth of a givenflutter mode, three narrow band peak filters per flutter mode could alsobe considered if phase loss in the disk-flutter servo is required to beminimized.

Next a comparison of the use of three narrow band peak filters to handlethe three components of the fundamental harmonic and the two side bandsversus a single 6 dB filter is plotted. It is important to note, thatgenerally narrow filters provide better phase properties. FIG. 18 showsthe comparison between a single 6 dB broad band filter and three narrow10 dB band filters. The computed phase lag at 700 Hz shows that a phaseloss improvement of 25% is achieved in the 3-filter configuration (8deg. vs. 6 deg.).

FIG. 19 is a set of plots that illustrates the measured power spectrumwith and without a disk-flutter servo correspond to FIG. 18 with thenarrow band filters. In particularly, FIG. 19a is a frequency domainplot of the power spectrum of the PES without any filtering, accordingto the present invention. FIG. 19b is a frequency domain plot of thepower spectrum of the PES with a three 10 dB peak filter set at 501 Hzin series the disk-flutter servo 208, according to the presentinvention. It can be observed that the 3-filter case shows as much ifnot better suppression of energy in the PES at the 501 Hz flutter modeas a single broad band filter as shown in FIG. 5. The dashed line inFIG. 19b represents and average and super imposed original spectrum fromFIG. 19a. Notice there is very little distortion of the spectrum of FIG.19a except at the frequency of interest, the 501 Hz flutter-mode.

Using the filter construction analysis from above for three 10 dB peakfilters, an adaptive filter architecture is implemented by usingthree-peak filters per disk-flutter mode. FIG. 20 is a block diagram ofan embodiment for determining the center frequency of a disk-fluttermode as in FIG. 11 but with the ability to determine bandwidth of thefundamental mode and each of the corresponding side bands 2002,according to the present invention. By employing a peak frequency andband width identifying operation as shown in FIG. 20, three peak filterscan be adaptively tuned to achieve the maximum performance of thedisk-flutter servo 208. Therefore, the flutter-mode filtering adjustedaway from 501 Hz +/−10 Hz to be adjusted over long term drift that isdue to normal operations of the hard disk drive 102. The drift can beaccommodated dynamically using the frequency band identifier 2002combined with (1) a dynamic filter with tunable center frequency 2004 or(2) a look-up table of previously cached measured valued for the modelof this hard disk drive 1021. The fundamental flutter mode frequency isshort term stable and does not require continuous adjustment of the peakfilter properties. But the side band filters will require much frequentor even continuous updates in order to track the flutter TMR errorresulting from the side bands. Using this method in FIG. 20, the filterconfiguration can be adaptively chosen to get the best performance outof a disk-flutter servo 208.

When the number of flutter modes is high, in certain configurations itmay be sufficient to increase the phase of the open-loop transferfunction so that rejection properties are balanced in such a way thatthe amplification of the flutter-induced disturbance is minimized. Thisservo characteristics is achieved by providing a lag-lead filter. FIG.21a is a block diagram of a lag-lead filter of an embodiment of thedisk-flutter servo system in FIG. 20 with a lag-lead compensator. FIG.21b is a frequency domain plot of the transfer function of a servosystem with flutter mode components in the range of 500-1000 Hz withouta lag-lead filter as shown in FIG. 21a, according to the presentinvention. FIG. 22c is a frequency domain plot of the transfer functionof a servo system of FIG. 22b with flutter mode components in the rangeof 500-1000 Hz with a lag-lead filter in position as shown in FIG. 21a,according to the present invention. In a particular hard disk drive 102,the flutter components existed in the range of 500-1000 Hz. For thishard disk drive 102, a lag-lead filter having corner frequencies at 100Hz and 200 Hz for lag portion, and 500 Hz and 900 Hz for lead portion isconstructed. Using this lag-lead filter it was possible to reduce therejection peak by about 5 dB in the 700-900 Hz region as shown in FIG.21. Observe the difference between the conventional servo and a servothat included the lag-lead filter as shown in FIG. 21. It was found thatthe reduction in low frequency gain accompanied by the use of alag-filter may need to be compensated by narrow band peak filters tomanage the low frequency periodic TMR sources. The use of lag-leadfilters in certain situations, can provide some benefits. Theflutter-mode is not amplified in the flutter TMR enhancement zone 2102.This shifting will not completely eliminate the flutter mode but in manycases reduces it and perhaps even eliminate the need for peak filtersall together. Generally, amplitude is more effectively localized overphase.

Turning now to FIG. 22, there is shown a block diagram of the majorelectrical components of an information processing 2200 system in whichthe flutter-servo system maybe implemented, in accordance with theinvention. The electrical components include: a central processing unit(CPU) 2202, an Input/Output (I/O) Controller 2204, a system power andclock source 2206; display driver 2208; RAM 22110; ROM 2212; ASIC(application specific integrated circuit) 2214 and a hard disk drive2218. a keyboard 2216 with a mouse 2232 receives the user input. Theseare representative components of a computer. The operation of a computercomprising these elements is well understood. Network interface 2220provides connection to a computer network such as Ethernet, TCP/IP orother popular protocol network interfaces. Optional components forinterfacing to external peripherals include: a Small Computer SystemsInterface (SCSI) port 2222 for attaching peripherals; a PCMCIA slot2224; and serial port 2226. An optional diskette drive 2228 is shown forloading or saving code to removable diskettes 2230 or equivalentcomputer readable media. The system 2200 may be implemented bycombination of hardware and software.

Although a specific embodiment of the invention has been disclosed, itwill be understood by those having skill in the art that changes can bemade to this specific embodiment without departing from the spirit andscope of the invention. The scope of the invention is not to berestricted, therefore, to the specific embodiment, and it is intendedthat the appended claims cover any and all such applications,modifications, and embodiments within the scope of the presentinvention.

Referring to FIG. 22, there is shown a block diagram of the majorelectrical components of an information processing system 2200 inaccordance with this invention. The electrical components include: acentral processing unit (CPU) 2202, an Input/Output (I/O) Controller2204, a system power and clock source 2206; display driver 2208; RAM2210; ROM 2212; ASIC (application specific integrated circuit) 2214 anda hard disk drive 2218. a keyboard 2216 with a mouse 2232 receives theuser input. Other pointing devices besides a mouse 2232 can besubstituted such as a trackball, joystick, glidepad, TrackPoint, andtouch screen. These are representative components of a computer. Theoperation of a computer comprising these elements is well understood.Network interface 2220 provides connection to a computer network such asEthernet, TCP/IP or other popular protocol network interfaces. Optionalcomponents for interfacing to external peripherals include: a SmallComputer Systems Interface (SCSI) port 2222 for attaching peripherals; aPCMCIA slot 2224; and serial port 2226. An optional diskette drive 2228is shown for loading or saving code to removable diskettes 2230. Thesystem 2200 may be implemented by combination of hardware and software.Moreover, the functionality required for using the invention may beembodied in computer-readable media (such as 3.5 inch diskette 2230) tobe used in programming an information-processing apparatus (e.g., apersonal computer) to perform in accordance with the invention.

Although a specific embodiment of the invention has been disclosed, itwill be understood by those having skill in the art that changes can bemade to this specific embodiment without departing from the spirit andscope of the invention. The scope of the invention is not to berestricted, therefore, to the specific embodiment, and it is intendedthat the appended claims cover any and all such applications,modifications, and embodiments within the scope of the presentinvention.

What is claimed is:
 1. A rotating media storage system comprising: arotating disk platter that exhibits a significant disk-flutter modecharacterized by a spectrum centered at approximately a first frequency,the disk-flutter mode being primarily caused by airflow-induced movementof the rotating disk platter normal to its surface; a read/writetransducer; and a servo feedback loop including a first peak filterhaving a filter band that includes the first frequency for providing aposition error signal to position the read/write transducer over therotating disk platter.
 2. The rotating media storage system according toclaim 1, further comprising: at least one narrow band filter with acenter frequency set to a frequency within the spectrum.
 3. The rotatingmedia storage system according to claim 2, wherein the narrow bandfilter is set to servo compensate a fundamental frequency of thesignificant flutter mode.
 4. The rotating media storage according toclaim 3, wherein the narrow band filter is set to at least one side bandfrequency of the significant flutter mode.
 5. The rotating media storageaccording to claim 2, wherein the narrow band filter is set to at leastone side band frequency of the significant flutter mode.
 6. The rotatingmedia storage system according to claim 1 wherein: the peak filtercomprises a lag-lead filter.
 7. The rotating media storage systemaccording to claim 6 wherein: the servo feedback loop further comprisesa narrow band filter with a center frequency within the spectrum.
 8. Aninformation processing system comprising: a rotating disk platter; aread/write transducer positioned over the rotating disk platter; and aservo feedback loop for providing a position error signal to positionthe read/write transducer over the rotating disk platter, the feedbackloop including at least one narrow band filter with a center frequencyset to at least one disk-flutter mode, the disk-flutter mode beingprimarily caused by airflow-induced movement of the rotating diskplatter normal to its surface.
 9. An information processing systemcomprising: a rotating disk platter; a read/write transducer positionedover the rotating disk platter; and a servo feedback loop for providinga position error signal to position the read/write transducer over therotating disk platter, the servo loop including a lag-lead filter tocompensate for disk-flutter-induced disturbances on the read/writetransducer, the disk-flutter being primarily caused by airflow-inducedmovement of the rotating disk platter normal to its surface.
 10. Theinformation processing system according to claim 9, further comprising:at least one narrow band filter with a center frequency set to at leastone flutter mode.
 11. A method of operating a rotating media storagesystem to reduce disk-flutter effects comprising the steps of: inducinga first disk-flutter mode characterized by a first spectrum centered atapproximately a first frequency on a disk platter by rotating the diskplatter, the disk-flutter mode being primarily induced by movement ofthe rotating disk platter normal to its surface due to airflow;providing a position error signal to a servo feedback loop; filteringthe position error signal with a first peak filter; and positioning aread/write transducer over the disk platter based at least partially onthe position error signal.
 12. The method according to claim 11, furthercomprising the step of: filtering the position error signal with anarrow band filter with a center frequency set to the flutter mode. 13.The method according to claim 12, wherein the step of filtering furthercomprises filtering the position error signal with a narrow band filterset to a fundamental frequency of the flutter mode.
 14. The methodaccording to claim 12, wherein the step of filtering further comprisesfiltering the position error signal with a narrow band filter set to aside band frequency of the flutter mode.
 15. The method according toclaim 11 further comprising the step of: measuring the first spectrum ofthe flutter mode.
 16. The method according to claim 15 furthercomprising the step of: setting a center frequency of the peak filter inaccordance with the first spectrum.
 17. The method according to claim 15further comprising the step of: setting a bandwidth of the peak filterin accordance with the spectrum.
 18. The method according to claim 11wherein the step of filtering comprises the sub-step of: filtering theposition error signal with a first peak filter that is characterized bya first center frequency that falls within the first spectrum.
 19. Themethod according to claim 18 wherein the step of filtering comprises thesub-step of: filtering the position error signal with a first narrowband peak filter that is characterized by a first center frequency thatfalls within the first spectrum.
 20. The method according to claim 19further comprising a step of: filtering the position error signal with asecond narrow band peak filter that is characterized by a second centerfrequency that falls within the first spectrum.
 21. The method accordingto claim 18 further comprising the step of: inducing a seconddisk-flutter mode characterized by a second spectrum centered atapproximately a second frequency on a disk platter by rotating the diskplatter; and filtering the position error signal with a second peakfilter that is characterized by a second center frequency that fallswithin the second spectrum.
 22. The method according to claim 21 whereinthe step of filtering the position error signal with as second peakfilter comprises the sub-step of: filtering the position error signalwith a second peak filter that is characterized by a second centerfrequency, a second transfer function, that falls within the secondspectrum, and overlaps a first transfer function that characterizes thefirst peak filter, such that a phase lag caused by the first peakfilter, is compensated by a phase lead caused by the second peak filter.23. The method according to claim 11, further comprising the sub-stepof: placing a narrow band filter set to filter out the fundamental modeof at least one flutter mode.
 24. A method of operating a rotating diskstorage system to reduce disk-flutter effects comprising the steps of:providing a position error signal to a servo feedback loop to position aread/write transducer over a rotating disk platter; and filtering theposition error signal with a lag-lead filter in series with the servofeedback loop to compensate for disk-flutter-induced disturbances on thetransducer, the disk-flutter being primarily caused by airflow-inducedmovement of the rotating disk platter normal to its surface.
 25. Themethod according to claim 24, further comprising the step of: filteringthe position error signal with at least one narrow band filter with acenter frequency set to at least one flutter mode.